My psychiatrist office mishandled my referral, and I was just sent a huge bill

A year ago, i got a referral from Kaiser to see a psychiatrist (or thats what I thought, because that’s what i requested). I saw this psychiatrist a few times, and never got billed for the entire year (maybe 4-5 sessions of seeing him).

A couple weeks ago, i had an appointment to see him again, but got a call from his office 2 days prior to the appointment, when i was told my referral was actually for psychology/therapy, not psychiatry/medication management. I checked with my insurance, and found out that the psychiatrist’s office was correct.

Fast forward a few days, my credit card gets billed $200. I emailed the office requesting the charge be cancelled, as i cancelled my appointment as soon as i got informed about the referral mixup (a few hours into the 2 business day cancellation period).

Now i just received a bill for $800+?! Wasn’t it their responsibility to check that they could accept my insurance? Who have they been billing this entire time, and how have they not noticed until now? Any suggestions on what to do next? Any advice would be greatly appreciated😔

TLDR; the psychiatrist’s office mishandled my referral, now theyre charging me $1000. What should I do? Who is responsible?